The present invention relates to treating agents and particularly to treating agents for filtration of caustic sodium aluminate solutions from the Bayer process.
In the Bayer process, alumina is refined from bauxite ores. The process comprises digesting the ore in a caustic solution to extract alumina, clarifying the liquor to remove caustic insoluble red mud material and precipitating alumina crystals as its trihydrate form from the clarified liquor. Clarifying the liquor involves separating the solid particles from the liquor by settling and if necessary, filtration.
The clarified, filtered liquor is then cooled until it becomes super-saturated and seeded with crystals of alumina trihydrate. Alumina is precipitated as the trihydrate in tanks and the solids are classified by particle size. The coarse fraction of alumina trihydrate (primary) is then dewatered eg by rotary vacuum filters and washed to reduce its soda level. The alumina product is then transferred to calciners where any free or combined water is removed if it is intended that the alumina be used in the manufacture of aluminium metal. If the alumina is to be used in other applications, some of the free water will be removed but not necessarily the water of hydration.
As discussed in U.S. Pat. No. 5,091,159 (incorporated herein by reference), it has been found that dextran is a particularly suitable treating agent for filtration. The dextran is added to the liquor, which preferably already contains a calcium aluminate filter aid, in quantities of 0.5-15 mg/l as the only treating agent. This addition of dextran has been shown to give substantial improvements in filtration time.
Of course, in the quantities used in the Bayer process, such additions of dextran can be quite expensive.
The present invention seeks to overcome at least some of the disadvantages of the prior art or at least provide a commercial alternative thereto.
In a broad first aspect, the present invention provides a treating agent for use in the Bayer process in filtration of caustic sodium alumina solution said treating agent comprising a blend of starch and another polysaccharide.
Preferably, the polysaccharide of the treating agent is selected from the group consisting of pullalan, dextran, alginate, zooglan, lactan or mixtures thereof.
In another embodiment, the treating agent is used in conjunction with a filter aid in the filtration of the caustic sodium aluminate solution. Preferably the filter aid is calcium aluminate and most preferably a 12% aqueous suspension of calcium aluminate.
Such a filter aid is useful when the caustic sodium aluminate solution is filtered by a Kelly pressure filter. Such a filter aid may not be required in all circumstances, eg, where a sand filter is used to filter the caustic sodium aluminate solution.
In a further preferred embodiment, the treating agent comprises up to 10 ppm preferably up to 5 ppm of the polysaccharide and up to 200-300 ppm of starch.
In still another preferred embodiment, the treating agent comprises a polysaccharide to starch ratio of between 9:1 and 1:9.
In a further aspect, the present invention provides an improvement in the Bayer process wherein bauxite is added to a caustic solution to produce a slurry of sodium aluminate solution and an insoluble red mud fraction which is then subjected to a thickener and separation step to produce a thickener overflow, said thickener overflow being subjected to filtration, the improvement comprising providing a blend of starch and another polysaccharide to the thickener overflow as a treating agent prior to said filtration.
The present applicant has surprisingly found that at least part of the polysaccharide normally used as a treating agent in the filtration step of the Bayer process can be replaced with starch without any substantial loss in filterability. Indeed the addition of certain blends of polysaccharide and starch as a treating aid may result in increased filterability of the liquor as compared to polysaccharide alone as a filter aid.
Clearly this has significant advantages since starch is only a fraction of the cost of the aforementioned polysaccharides. Any replacement of the polysaccharide by a low-cost product such as starch will substantially increase the commercial viability of the process.
Unless the context clearly requires otherwise, throughout the description and the claims, the words xe2x80x98comprisexe2x80x99, xe2x80x98comprisingxe2x80x99, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of xe2x80x9cincluding, but not limited toxe2x80x9d.